Control of heat absorption in steam superheater



F. l. EPLEY Sept. 3, 1957 CONTROL OF HEAT ABSORPTION IN STEAM SUPERHEATER Filed Sept. 21, 1951 F/fofR/c .g EpLe-y IN V EN TOR.

United Sttes CONTROL OF HEAT ABSORPTIN 1N STEAM SUPERHEATER Frederic I. Epley, Rockville Centre, N. Y., assigner to Combustion Engineering, Inc., New York, N. Y.

The present invention relates to steam generators and particularly to an improved arrangement for the control of the final temperature of steam delivered by a superheater, or reheater, associated with a steam boiler.

Heretofore, various arrangements have been resorted to for regulating the temperature of steam delivered by a superheater associated with a steam generating boiler. Conventional arrangements include the use of dampers for by-passing around the superheater part of the gases owing thereto from the boiler furnace or the use of a desuperheater. One such arrangement comprises the division of the superheater into several sections with a desuperheater located between them in the steam flow path in order to maintain the iinal steam temperature to a desired value. While both of these mentioned arrangements, and others not specifically described herein, have atent certain advantages, they also possess certain features which are sometimes considered objectionable.

The object of the present invention is to overcome the disadvantages entailed by the use of by-pass dampers or desuperheaters as well as to eliminate the expense attendant upon the provision and use of such apparatus for controlling superheat temperature. The present invention contemplates the control of the final temperature of steam from a superheater by controllably regulating the temperature of gases that flow over a heat absorbing section of the superheater or reheater. Specifically, this contemplates the employment of another heat exchanger, such as a part of an economizer, in a location where more or less heat may be abstracted from the gases before they flow over the heat absorbing elements of a superheater section or a reheater.

The invention Will be best understood upon consideration of the following detailed description of an illustrative embodiment thereof when read in conjunction with the accompanying drawing in which the single figure is a diagrammatic illustration of a steam generating boiler equipped with a superheater and utilizing an economizer section to control superheat temperature in accordance with the invention.

In the drawing the numeral 10 designates a boiler furnace in which fuel is burned for the generation of steam as in the water tubes 12 aligning the walls of the furnace. The mixture of steam and water is discharged into a separator drum 14 and the separated steam flows through a conduit 16 to the inlet header 17 of the superheater which is located in a down pass 18 in communication at its upper end with the outlet of the furnace 10.

According to the invention the heat absorbing elements of the superheater that are located in the down pass 1S are divided into a primary section 20 and a secondary section 22 which are spaced apart in the direction of gas ilow. In some installations an additional superheater section 23 may be provided between the outlet of furnace 10 and the down pass of furnace 18. Y In the arrangement shown, the steam to be superheated passes in counterow rst through the primary section 20 and then in series through the secondary section 22 from which it is discharged into the preheater outlet header 24. Section 23 may follow the section 22 in series. Located in gas pass 18 between the primary section 20 and the secondary section 22 of the superheater is a heat exchanger which, in accordance with the invention, has its heat absorbing capacity controllably regulated so as to absorb more or less heat from the gases after they have passed over the secondary section 22 in order to predeterminately regulate the temperature of gases that ow over the primary section 20 of the superheater.

Although various types of heat exchangers might be employed between the two sections 20 and 22 of the superheater an economizer is chosen for the purpose of illustration. The feed water for the boiler is provided by a supply line 25 that is divided into the branches 26 and 27 having valves 28 and 29 therein for regulating the apportionment of the feed water between a main economizer section 30 and another economizer section 32 which is positioned in the location between the primary section 20 and the secondary section 22 of the superheater. The heated water from the economizer sections passes through the pipe 33 into the steam and water drum 14. The water flow path through the economizer section 30 constitutes a by-pass around the control section 32. A thermostatic device 34 responsive to the temperature of steam in the superheater outlet header 24 is arranged to control the operation of the valve operating mechanisms 36 and 38 in conventional manner to operate the valves 28 and 29 so as to apportion the ow of feed water between the economizer sections 30 and 32 in such manner as to maintain a predetermined final steam temperature.

The section 22 (or 22'and 23) of the superheater has sufficient heat absorbing surface to give the required steam temperature at maximum capacity of the steam generating unit. There is then no need for obtaining heat from the section 20 of the superheater. To accomplish the elimination of a superheating eect in section 20 the economizer part 32 is made suiiiciently large to reduce the gas temperature entering section 20 to the point where it would equal the saturated steam temperature entering the superheater.

On the other hand, when the load on the boiler is reduced, the gas temperature Ta is reduced, thereby reducing the amount of heat that section 22 (or 22 and 23) may absorb. ln order to maintain desired iinal steam temperature leaving the superheater outlet header 24, section 2i) is made eifective and this is done by reducing the amount of water flowing through the economizer section 32, even to the extent of eliminating the water flow completely. The diversion of the water is accomplished by means of the valves 28, 29 directing the llow of water to or away from the economizer section. It is of course important that the economizer section 32 be located in a gas temperature zone which will not be excessive at times when no water passes through the economizer tubes.

Thus, the steam temperature delivered by a superheater may be controlled by eifectively controlling the difference between the gas temperature and steam temperature at the cold end of the superheater surface. The heat absorbing surface in section 22 (or 22 and 23) would be xed so as to obtain the desired steam temperature at full load. The economizer surface 32 is made sucient to reduce the gas temperature, T2, to the same level as the saturated steam temperature so as to render the surface in superheater section 20 ineffective. As the load diminishes, gas temperature, T3, becomes lower so as to require more superheater surface. At half load for example,

-the economizer surface 32, is made ineffective by bypassing water through the main economizer 30 so that superheater surface 2t) becomes fully effective. Surfaces 22 (or 22 and 23) plus section 20 are sulicient to give full steam temperature at reduced loads.

'1 The same principle can be applied to reheat boilers to control both primary and low pressure steam temperatures. In a boiler equipped with a reheater the latter might be mounted in the place of the primary section of the superheater. The reheated steam temperature would then be Vcontrolled in the manner described above,l as will be apparent, since a reheater is essentially'a steam superheater but used for raising the temperature'of steam from the primary section of a turbine before it continuesr to the secondary turbine.

What I- claim is: Y

l, In fluid-heating apparatus having a gas pass through which hot gases ow;fsuperheater means including a superheater portion disposedin said pass; a lipid? heater having heat absorbing elements disposed in saidgaspass ina position upstream of said superheater portion` with respectto the direction of gas ow in said pass; means for circulating a liquid to be heatedthroughsaid heat absorbing elements with the heat absorbed by the,l liquid varying with its rate of flowthrough the heater; and means for controlling the flow of liquid throughsaid. heater, said means being regulated inresponse to the superheatedtsteam temperature to increase and decrease the ow through the fluid heater. in response-'to an increase and: decrease respectivelyk of this'. temperature wherebyn said means is etective tor control the heat; absorbed by said liquid so as to maintain a predetermined temperature at the4 outlet of` saidr superheater` 2. In fluid heating apparatus. havinga gas, pass through which hot gasestow.; iirstl and second steam superheater sections disposedin said pass andl spaced apart in the direction of ilow of gases therethrough; a fluid heater having heat absorbing elements disposed in said gas pass in a postionbetween the two superheater` sections; means for circulating a liquid, to be heated through` said heat absorbingelements with therheat absorbed by the liquid varying'with its rate of flow through the heater; a by-pass conduity so connectedrasto divert part of the liquid to be heatedaround said heat absorbingelements; and means forV regulating and apportioning the flow of liquid between said heater and said by-pass so as to maintain a predetermined steam temperature at the outlet end of the second superheater section.

3. In a boiler having steam. generating elements and a fuel burning furnace and having an outlet. from which the products of combustion flow through a gas passageway; a steam superheater connectedto receive steam generated in said steam generating elements, said superheater being divided; into rst and second sections disposed in saidpassagewayand connected forvseries dow of steam therethrough in the direction opposite to the flow of gases through said passageway; a heat exchanger having heatabsorbing elements disposed in said gas passageway in a position between the two superheater sections; means for circulatinga lluid to be heated through said exchanger elements; a by-pass conduit so connected as to divertpart of the fluidtotbe heated around said exchanger elements; meansfor regulating and apportioning the flowv of fluid between the said exchanger elements and said by.pass; andmeans responsive to the temperature of superheated steam at the outlet of said superheater operative tocontrol said regulating means to increase and decrease the flow through the heat exchanger elements in responseto an increase and decrease respectively of said temperature to maintain a predetermined superheated steamv temperature.

4; lnaboiler having a steam and water drum, steam generating elements connected thereto, and a fuel burning furnace having an outlet from which the products of combustion flow through a gas passageway; a steam superheater connected to receive steam generatedl in said steamY generating elements, said superheater being divided intok two series: connected sectionsl disposed in said passageway in spaced apart relation With respect to the flow of gases through said passageway; awater heating economizer having heat absorbing elements disposed in said gas passageway in a position between the two sections of said superheater; means for circulating Water through said economizer elements to said drum; a by-pass conduit so connected as to divert part of the water around said economizer elements; and means for regulating and apportioning the flow of water between the elements' of said economizer and said by-pass operative to maintain a predetermined superheated steam temperature,

5. In a boiler the combination of a furnace into. which fuel isintroduced and burned, means associated with said furnace for generating steam, a gas passageway communicating with said furnace so that the combustion gases generated within said furnace are conveyed therethrough, means forming a part of the boiler for superheating the steam generated in said steam generating means by absorbing heat from said combustion gases and including a heat exchanger disposed within said gas passageway, means for supplying heated feedv water to said boiler comprising an economizer'including heat exchange means disposed in saidl gaspassageway upstreamof said superheater heat exchanger with respect to combustion gas ow, and heat exchange means disposed in -said gas passageway downstream of said superheater heatf exchanger with respect to combustion gas flow with said upstream and downstream heat exchange means being in parallel with respect to the ow of feed water, and means operative toadjustably proportion the flow of feed water between said upstream and downstream heat exchange means to vary the heat in said combustion gases available tok heat the steam passing through said superheater heat exchanger.

6. In aboiler the combination of a-furnaceinto-which fuel is introduced and burned, means associated with said furnace for. generating steam, a gas passageway communicating-withsa'id furnaceso that the combustion gases generatedzwithinsaid furnace are conveyed therethrough, means forming a part of the boiler for superheating the steam generated insaidsteam generating means by absorbing heat from said combustion gases and including a heat exchanger disposed within said gas passageway,

meansfor supplying heated feed water to said boiler comp rising aneconomizer including heat exchange meansdisposedA in said vgas passagewayupstream ofy said superheater heat exchanger with: respect to combustion gas low, and heat, exchange meansL disposed in said gas passageway` downstream of said superheater heat exchanger with respect to combustion gas ow with saidvupstream and downstream heat exchange means beingin paralleliwith respectto the dow of` feed water, the location of said upstream heat exchange means vbeing such that it willnotb'e deleteriously affected if thel flow of feed water'thereto should be completelyl stopped'at and below a predetermined load on` the boiler within the normal operating rangeof the boiler, and means operative to adjustably pro-I portion the flow of feed water betweensaid upstreamiand downstream heat exchange means to vary the heatingsaid combustion gases available to heat the steam passing throughsaid superheater heat exchanger.

References Cited in the le of this patentA UNITED STATES PATENTS 1,753,363` Coghlan et al. V Apr. 8 1930 2,222,769 Hardgrove Nov. 26, 1940 2,579,027 Walter et al; Dec. 18,' 1951 2,590,712- Lacerenza c Mar. 25, 1952 FOREIGN PATENTS 835,307 France Sept.- 19, 1938 912,231 France Apr. 23, 1946 2,963` Great Britain c 1902 new.; cxx., 

